The present invention relates to an image sensor driving device, and more particularly to an image sensor driving device wherein an S/N ratio of an original information read out signal obtained from an image sensor is improved.
FIG. 1 is a circuit diagram illustrating a prior art image sensor driving device. In the figure, the driving device includes M blocks of photo-cell sections B(1) through B(M), in which each block has N pieces of photo-cells; switching elements 2(1) through 2(MN), each connected to one terminal of the associated photo-cell; N pieces of signal lines L(1) through L(N), each signal line connected to one of the switching elements in each block; and an analog multiplexer 10, the inputs of which are connected to signal lines L(1) through L(N). The other terminals of the respective MN pieces of the photo-cells are connected to a power supply VB through a common line CL. The switching elements are connected on a block basis to the respective common gate lines G1, G2, ---, G(M).
The analog multiplexer 10 include, as shown in FIG. 2, a group of reset switches 4(1) through 4(N), each being formed with an FET which is connected between each one of the associated signal lines L(1) through L(N) and ground; voltage follower type amplifiers 3(1) through 3(N) each having high input impedances; and a group of switches 5(1) through 5(N) connected respectively to the outputs of the amplifiers 3(1) through 3(N) and on-off controlled by outputs of a shift register 11.
In the image sensor driving device thus arranged, when light reflected from the original is irradiated onto the photo-cell sections B(1) through B(M), each photo-cell causes a capacitor connected in parallel thereto to discharge in accordance with the amount of light incident to the photo-cell and to convert the original information into an amount of electric charges being stored.
The reset switches 4(1) through 4(N) are turned ON at appropriate timings to reset the signal lines L(1) through L(N), and are then turned OFF. When the gate line G1 is supplied with a gate voltage, switching elements 2(1) through 2(N) turn ON so that voltage information corresponding to the storage of electric charges is conveyed to the signal lines L(1) through L(N). Thereafter, switches 5(1) through 5(N) of the analog multiplexer 10 are successively turned ON in response to the output of the shift register 11, thereby successively connecting the signal lines L(1) through L(N) to an output line OUTL. With this operation, the original information in the photo-cells 1 through N within the first block B(1) is amplified by the amplifiers 3(1) through 3(N) and is read out from the output line OUTL, respectively.
Following this read-out operation, the voltage applied to the gate line G1 is set to a low level. After the reset switches 4(1) through 4(N) are turned ON, the gate line G2 is supplied with the gate voltage. With such an operation, the switching elements 2(1) through 2(N) are rendered OFF whereas the elements 2(N+1) through 2(2N) are rendered ON. Hence, the original information sensed by the second block of the photo-cell section B(2) is transmitted to the signal lines L(1) through L(N). The original information covered by the second block B(2) is transmitted to the output line OUTL by the analog multiplexer 10.
With a repeat of the same operation, the original information for one line sensed by the photo-cells of the remaining blocks is successively read out to the output line OUTL.
The prior art driving device as described above has problems which will be described hereinafter.
In deriving the original information sensed by the photo-cell from the output line OUTL, the gates of the reset switches 4(1) through 4(N) are supplied with a reset signal to turn ON the reset switches 4(1) through 4(N). This is performed to remove residual information of the previous block from the signal lines L(1) through L(N) after reading the original information is one block but before reading the original information in the subsequent block.
However, ON/OFF operation of the reset switches 4(1) through 4(N) cause to produce noise. Due to the noise on the signal lines L(1) through L(N), an S/N ratio of the information is deteriorated.
In addition, despite the fact that an input signal level to the analog multiplexer 10 would be 1 through 100 mV and a gain of the individual amplifiers 3(1) through 3(N) would be 100 times, variation in offset of each amplifier is several tens mV. Further, the variation in the gain is in the range of approximately .+-.50% according to the variation in each value of resistors r1 through rn and R1 through Rn. This makes it difficult to obtain an excellent S/N ratio.
In addition, if the centuple (100 times) gain amplifier is replaced by cascade-connected two-stage amplifiers, each being decuple (10 times) gain, influence of the variation in the resistor values become small. Yet, it is still impossible to remove the offset of the amplifier and hence the problem can not be solved.